Programmable logic devices (PLDS) are a class of integrated circuits which can be programmed by a user to implement user-defined logic functions. PLDs are often used in electronic systems because, unlike custom hard-wired logic circuits or application specific integrated circuits (ASICs), PLDs can be programmed in a relatively short time and can be reprogrammed quickly to incorporate modifications to the implemented logic functions. One major class of PLDs are referred to as programmable logic array (PLA) devices or programmable array logic (PAL) devices. Early PLDs include a group of AND gates which logically ANDs two or more input signals to produce product terms (P-terms), and a group of OR gates which logically ORs two or more of the P-terms generated by the AND gates. The AND gates are typically formed as a matrix of programmable connections where each column connects to an input pin of the PLD, and each row forms a P-term which is transmitted to the group of OR gates. The OR gates may be programmable (i.e., each P-term is programmably connectable to one of several different OR gate outputs), in which case the PLD is referred to as a PLA device. Alternatively, the OR gates may be fixed (i.e., each P-term is assigned to a particular OR gate output), in which case the PLD is referred to as a PAL device. The AND gates and OR gates of PLA and PAL devices implement logic functions represented in the sum-of-products form.
PLA and PAL devices were well-received by logic designers when their implemented logic functions were relatively small. While PLA and PAL circuits are commonly used in PLDs, they may also be used in any other type of integrated circuit, such as an application specific integrated circuit (ASIC) which also has fixed hardware. However, as logic functions have grown increasingly larger and more complex, logic designers were required to couple together two or more small PLDs to provide sufficient logic capacity. Although this process was tolerated during development and testing, it increased the cost and size of production units. This generated a demand for PLDs with increasingly larger logic capacity. To meet the ever-increasing demand for greater capacity, PLDs with increasingly complex architectures have been developed. One popular complex PLD type, known as complex programmable logic devices (CPLDs), includes two or more function blocks connected together and to input/output (I/O) modules by an interconnect matrix such that each of the function blocks selectively communicates with the I/O modules and with other function blocks of the CPLD through the interconnect matrix.
Another type of PLD is a field programmable gate array (FPGA). In a typical FPGA, an array of configurable logic blocks (CLBs) is coupled to programmable input/output blocks (IOBs). The CLBs and IOBs are interconnected by a hierarchy of programmable routing resources. These CLBs, IOBs, and programmable routing resources are customized by loading a configuration bitstream, typically from off-chip memory, into configuration memory cells of the FPGA. For both of these types of programmable logic devices, the functionality of the device is controlled by configuration data bits of a configuration bitstream provided to the device for that purpose.
Because CPLDs typically use sense amp technology and other circuits that draw current even when static, conventional CPLDs may unnecessarily draw current. To minimize the current, chip designers transfer internal EPROM bits to internal latches called “configuration bits.” After the bits transfer, the EPROM is powered down. However, there is still current drawn by other circuits of the CPLD. Further, digital systems often combine a myriad of chips including PLDs with different voltage configurations. For example, designers must interface 2.5V processors with 3.3V memories, both RAM and ROM, as well as 5V buses and multiple peripheral chips. Therefore, a CPLD may require multiple internal voltages.
Accordingly, there is a need for an improved circuit for and method of minimizing power consumption in an integrated circuit device.